In many industries it is important, and indeed even crucial, to monitor the conditions of a liquid within a housing or container. Monitoring of the liquid conditions is typically achieved using one or more gauges which extend into the housing and into contact with the liquid. However, in the semiconductor industry especially, and in other industries as well, extremely severe conditions are often times encountered within liquid containers due to the highly corrosive nature of many liquids.
Corrosive liquids are extremely hard on many gauges because metals which are used in conventional gauges cannot reliably withstand the corrosive environment for long periods of time. Thus, not only must conventional gauges be frequently replaced, thereby increasing costs, the processing equipment associated with the container must be shut down during gauge replacement, thereby affecting production and further increasing costs.
Conventional sensors are disclosed in U.S. Pat. Nos. 5,501,102 and 5,626,053 to Williamson. In these sensors, a pair of non-metallic, resistive/conductive members are provided in order to prevent corrosion by the corrosive environment within the container. The resistive/conductive members are formed from conductive, chopped carbon fibers embedded within a polymeric material. These sensors rely upon either a change in voltage or a change in current between the two members, which are in direct contact with the liquid, to provide an indication of the liquid level in the container. The resistive/conductive members, however, have a relatively high resistance, due to the chopped carbon fibers, which do not electrically align during formation of the members.
Another conventional sensor device is disclosed in U.S. Pat. No. 5,057,813 to Sasaki et al. This device is a digital output device for sensing whether a liquid, in this case oil, falls below a predetermined level. A metallic electrode is disposed inside of a sensor body, and the resistance between the sensor body and the electrode is detected to determine the oil level. This device, however, is not able to continuously sense the oil level. Further, due to the metallic nature of the electrode, this device would not be practical for use in highly corrosive environments.
U.S. Pat. No. 5,661,405 to Simon et al. discloses a sensor for us in electrically conductive liquids. The sensor uses two flexible ribbon electrodes surrounded by a pair of flexible sleeves. Each electrode is made from a carbon filled, silicon rubber material. Silicon rubber material offers very limited corrosion resistance and is therefore not suited for use in corrosive environments. Further, silicon is forbidden in the presence of silicone wafers during semiconductor manufacturing, and thus this sensor is not suitable for use in the semiconductor industry.
Therefore, there is a need for sensors that overcome the disadvantages of previous sensors. Further, there is a need for improved sensors for use in highly corrosive media, such as those used in the semiconductor industry, for sensing the conditions of the media.